The idea, then, is to have the OSS engine ... and find someone with big pockets to write the forms. I'm kinda thinking of a generic tax package (hopefully, generic enough to fit any country), but with "localisation" packages that describe fields, and the relationships between fields, etc. The challenge (well, the biggest challenge) would be getting someone to write the localisation package every year for each country. Ideally, that would be the government of that country (or at least the tax agency of the country).

The engine would then produce an XML file as output, and the localisation package would include an XSLT transform that would create the country-specific format (whether that's an HTML file to print out, or it's a specific on-line submission version, such as what Canada does).

This would be a challenge in that many less-socialist countries would not want to compete with the private sector for something like this. The engine would be fun to write ... but getting someone to sign on for the country-specific parts would be quite difficult.